1. Field of the Invention
This invention relates to an evoked potential measuring apparatus, and more particularly to an evoked potential measuring apparatus by which an evoked potential which reflects a variation in internal condition of a human being can be extracted in a high quality and high reliability.
2. Description of the Related Art
When some stimulation is applied to a sense of a human being, a corresponding brain potential is evoked at an information processing location of the brain which takes charge of the sense. The evoked brain potential is called evoked potential and is measured by way of electrodes from the head skin on the information processing location of the brain corresponding to the sense. An evoked potential from a stimulus to a sense is used frequently in fundamental experiments of physiology, psychology and biotechnology.
An exemplary one of methods for detecting an evoked potential is disclosed in T. Yamazaki, K. Kamijo and S. Fukuzumi, "QUANTITATIVE EVALUATION OF VISUAL FATIGUE ENCOUNTERED IN VIEWING STEREOSCOPIC 3D DISPLAYS: NEAR-POINT DISTANCE AND VISUAL EVOKED POTENTIAL STUDY", Proceedings of the Society for Information Display, Vol. 31, No. 3, 1990, pp.245-247. According to the method, a visually evoked potential, which is used effectively to solve the problem of fatigue, is measured for a certain period of time after a point of time at which a light is emitted as a stimulus using a flash apparatus as a light emitting element. The potential evoked at the head skin on the information processing location of the brain corresponding to the visual sensation by the stimulus of emission of light ranges ten and several microvolts to several tens microvolts. The potential is lower than a potential arising from activities from any locations of the brain than the information processing location corresponding to the visual sensation, and the potential obtained by measurement by a single stimulus may be a potential influenced by another other location of the brain than the information processing location corresponding to the visual sensation. Therefore, a similar measurement is repeated by a plurality of times, and potentials at an equal time after emission of light are added for the individual times and an average value of the potentials is calculated. The adding and averaging processing is based on the ground that, if an influence upon each measured potential of an potential from the other portions of the brain is regarded as random noise, then if the measured potentials are cumulatively added, then the noise can be reduced to zero.
An exemplary one of conventional evoked potential measuring apparatus is disclosed, for example, in T. Nakamura and Y. Morita, "Spectral sensitivity characteristics of visually evoked potentials (VEP)", Technical Report of the Television Society of Japan, Vol. 10, No. 48, 1987, pp.1-6. The evoked potential measuring apparatus has such a basic construction as shown in FIG. 6. Referring to FIG. 6, a visual stimulus is presented from a visual stimulus presentation section 41 in synchronism with a signal outputted from a pulse generator 44. A potential evoked when the presentation is visually sensed is inputted to an amplifier 42 by way of an electrode 47. The waveform of the signal amplified by the amplifier 42 is displayed on a monitor 43 so that an experimenter may visually observe it. Further, the amplified signal is added in an analog fashion to such signals in the past by an adder 45 in synchronism with the signal outputted from the pulse generator 44. A result of the addition is recorded as a graph onto a record paper sheet by an X-Y recorder 46.
The present day society is called a society of stress, and patients of diseases caused by stress arising from fatigue or complicated human relations are progressively increasing, for example, in places of work. Since the cause of stress lies in one's own mind, the stress originates in the function of the brain. Accordingly, also the evoked potential is considered to be/as an effective means. The visually evoked potential is effective to detect VDT (Visual Display Terminal) fatigue. However, a visually evoked potential detected by any conventional technique merely reflects the function of the visual sensation, but does not directly reflect a variation of a higher order central nervous system hidden behind a drop of the function of the visual sensation. In other words, an evoked potential detected by any conventional technique which makes use of a visual stimulus is not a brain potential which reflects a feeling of fatigue, a feeling of stress, an irritated feeling or a feeling of strain itself.
Further, even if a stimulus which induces stress is found out, any conventional technique which involves adding and averaging processing is disadvantageous in that a brain potential which reflects the function of a sense may possibly be included in a measured brain potential and it is difficult to extract only a brain potential which reflects a higher order center function.
In summary, with the conventional evoked potential measuring apparatus described above, while it is possible to detect a brain potential which reflects various functions of senses, it is impossible to measure a brain potential which relates a function of a higher order central nervous system for a feeling of strain, a feeling of irritation, a feeling of stress and so forth.